The invention relates to a cylinder head gasket having two surface plates and a bead plate, more particularly, a cylinder head gasket capable of generating an optimum surface pressure with respect to portions where heat load is high in a peripheral portion of a cylinder bore.
As a gasket for sealing joint surfaces between a cylinder head and a cylinder block of an internal combustion engine, there has been used a cylinder head gasket formed by laminating thin metal plates on which a sealing device is formed along a periphery of a hole to be sealed.
In view of demands for making an internal combustion engine small and light weight, especially a small type diesel engine, engine members, such as a cylinder head and a cylinder block, have been made of an aluminum alloy or the like with a relatively low rigidity. Since the engine has been designed to have a high output, as the pressure (Pmax) in a cylinder is increased, a high sealing ability has been required.
While satisfying the requests of the engine, to provide a proper sealing ability to a gasket, it is necessary to provide not only a first sealing device but also a second sealing device along the peripheral portion of a cylinder bore. However, to meet the demands of the small size, light weight and low cost of the engine, the number of the gasket plates to be laminated has been decreased and the sealing device to be employed has been simplified. As a metal laminate type gasket having a relatively simple structure, a metal laminate gasket including two surface plates and an intermediate plate has been practically used.
There has been proposed a metal laminate gasket including three metal plates, wherein with respect to a cylinder bore, a first sealing portion by a first bead and a second sealing portion by a second bead disposed outside the first bead are provided; and one of surface plates is folded back to protect the first and second beads from a combustion gas of a high temperature.
As an example of the gasket described above, as shown in FIGS. 9(a) and 9(b), Japanese Patent Publication (KOKAI) No. 11-118037 discloses a metal laminate type gasket 60, wherein a first outer plate 61 is folded back to the side of a second outer plate 62 to form a folded-back portion 65 for constituting a cylinder hole 64; a first bead 66 surrounding the cylinder hole 64, provided on the second outer plate 62, inside the folded-back portion 65; and an intermediate plate 63 disposed between the first and second outer plates 61, 62 outside the folded-back portion 65, and provided with a second bead 67 surrounding the folded-back portion 65. In the gasket, the tightening load is unified by changing a folded-back width LX, bead widths BX1, BX2 and bead positions RX1, RX2 according to the distance from a tightening bolt hole 68 to thereby prevent the bore from being deformed and to improve the gas sealing ability. Incidentally, in the embodiment shown in FIGS. 9(a) and 9(b), a coating film 70 is applied on both sides of the intermediate plate 63.
However, in the metal laminate type gasket 60, the intermediate plate 63 is disposed outside the folded-back portion 65, so that the space between the first bead 66 and the second bead 67 becomes large. Thus, there has been a problem such that, in view of the tendency of making the engine small to allow the space between the cylinder bores to become narrow, it is hard to apply the gasket to the recent engine.
Also, there has been a problem such that, in order to distribute the tightening force uniformly by tightening the bolts, deterioration of sealing ability due to heat load and heat stress, which have recently been a problem, is not sufficiently dealt with.
As the pressure in a cylinder (Pmax) of a diesel engine becomes high, the problem of the deterioration of the sealing ability by the heat load and heat stress has become the public attention. The problem arises at, for example, a portion on the central side or the exhaust manifold side of the engine where the temperature is relatively high and the heat load becomes large; or a portion where the heat load locally becomes large due to the circulation of the cooling water or the like, for example, the end portion and the exhaust side of the engine.
On these portions, the sealing surface pressure of the bead portion and the like becomes high due to increase in the heat stress by the heat load. Also, a large force is repeatedly applied to the large sealing surface pressure portions due to increase in a relative displacement, i.e. head lift, between the head gasket and the engine member during the operation of the engine. Thus, pressing marks on the engine member against which the large sealing surface pressure portions abut are increased. In other words, the engine member is locally depressed to thereby lower the gas sealing ability.
Especially, in the engine member having a relatively low rigidity as in an aluminum engine, in case a bead having a high rigidity is provided to the gasket, when the engine member is hit by the head lift vibration during the operation of the engine, the hit portion is depressed.
On the other hand, to prevent the problem, when the rigidity of the bead is lowered too much, the sealing ability is reduced. Also, since the tightening forces by the tightening bolts are fixed, it is necessary not to cause the leakage by uniformly distributing the tightening force for pressing the engine members, balancing the surface pressures of the respective sealing portions, i.e. not to cause the depressions on the engine member, and controlling to obtain sufficient surface pressures for sealing.
In view of the above problems, the present invention has been made and an object of the invention is to provide a cylinder head gasket for sealing between a cylinder head and a cylinder block, wherein rigidity of a bead is lowered with respect to portions where high surface pressure is generated by large heat load and heat stress at high temperature portions, so that the press marks on the engine member are reduced to thereby hold a high sealing ability.
Further objects and advantages of the invention will be apparent from the following description of the invention.